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Discussion Starter · #1 ·
I have a friend with a pool hall and I want to help him set up a projector. The room has two floors with the second floor extending out over half of the room. The position of the screen would in the corner of the high ceiling (about 30 ft high) half of the room so the screen could be seen from both floors.


There is plenty of room for a 200 inch screen. I think he needs to go a minimum of 120 inches.


There will be some ambient light from the pool table lights and other low level indirect lighting.


My question is how to determine the projector brightness needed for a given screen size and the amount of ambient light?


Given I measure the light with a meter, can I plug that into an equation along with screen size and solve for projector lumens?


--sdc

 

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I saw this some time ago in the screens forum in a post by Don Stewart (of Stewart Filmscreens):


(projector lumens/screen square feet) * screen gain = Ft-Lamberts


or, solving for projector lumens:


lumens = (ft-lamberts * sqft) / gain


He also suggested the ft-lamberts for a perferated screen should be ~20 and non-perferated ~17.


Unfortunately, it doesn't include ambient light and was in a discussion of a light controled HT.


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Huck
 

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Here are some calculations, where:


lumens = projector’s brightness in ANSI lumes

ftL = screen brightness in foot lamberts

gain = screen gain

diagonal = diagonal screen size in inches given a 16:9 (1.78:1) aspect ratio



1. You have a screen with a given size and gain and you want to know how bright your projector needs to be for your desired screen brightness:


lumens = ( diagonal x diagonal x ftL ) / ( gain x 337 )



2. You know how bright the projector that you are going to use is and how big you want your screen to be, but you want to figure out what gain you need to achieve your desired screen brightness:


gain = ( diagonal x diagonal x ftL ) / ( lumens x 337 )



3. You know how bright the projector that you are going to use is and what gain the screen material that you want to use has, but you want to figure out how large you can go on the screen while still achieving your desired screen brightness:


diagonal = square root [ ( lumes x gain x 337 ) / ftL ]



Key to using the formulas is to figure out your desired screen brightness. Ideally, you want to set this constant so high as to achieve good picture dynamics and overall punch. What such a threshold could be is an answer I am looking for myself, but I wouldn’t try lower than 30 ftL, preferably +40 ftL.


Maybe someone can provide a link to a post where this magical threshold has been discussed already or provide valuable insight on this matter.


[This message has been edited by NiToNi (edited 10-10-2001).]
 

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Steve - Here is where I got the formula I posted.


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Huck
 

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Steve,


Note that "sq. diagonal" means "diagonal x diagonal". I'll edit the formulas as to make this crystal clear.


An example for each scenario/formula to illustrate their use:


Formula 1: You have a Grayhawk 100" 16:9 with a gain of 0.95. Your target screen brightness is 30 ftL. You projector's brightness need to be (100x100x30)/(0.95x337)=300,000/320.15 = 937 ANSI lumens.


Formula 2: Your PJ outputs 800 ANSI lumens and you would like to use a 110" screen. Your target screen brightness is 30 ftL. Thus, you need a screen gain of (110x110x30)/(800x337)=363,000/269,600 =1.35


Formula 3: You want to use the new Sharp XV-Z9000U (800 ANSI lumens) on a Grayhawk (gain 0.95). You want the picture to be watchable with some ambient light present while enjoying that extra "umph" when you watch in the dark, so your target screen brightness is 40 ftL. The largest (diagonal) you can go with a 16:9 screen then is the square root of (800x0.95x337)/40 = 80".


Re ambient light, you need to set your target screen brightness as to allow for viewing under such conditions.
 

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Hmm. Well #3 is basically my desired setup. See my other thread: www.avsforum.com/ubb/Forum10/HTML/009519.html


However, based on your calculations, a 100" diagonal isn't going to be bright enough for me. However, other people state that they have seen this combination and that it is great...And the Stewart guy claims you need less ft-lamberts....Frustrating ain't it? Don't know who to believe...Arg.


-Jon


 

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Acceptable brightness in a high ambient environment is VERY subjective. A formula wont help much. Better to set up a temporary projector/screen and vary the distance to the screen until you find the brightness is satisfactory. Then calculate the screen lumens and use that as a rough guide for the final larger setup.


If you can't borrow a projector, even an RPTV or direct view set could be used. Just adjust contrast until the brightness is adequate then measure with your light meter.


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currahee
 

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Discussion Starter · #9 ·
NiToNi,Huckster,

This is very useful information but the equations given do not match each other. It takes approximately double the ftL in what NiToNi posted to equal what Huckster posted. Can anybody comment on this discrepancy?



It would be nice to get something that incorporates ambient light. Given a measured illuminance of ambient, and knowing ftL, gain, and diagonal I want to compute lumens.


Anybody know how to add lumens?


--sdc

 

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Steve - You have paraphrased the ONLY true statement in this crazy hobby. http://www.avsforum.com/ubb/smile.gif


Try it/see it/listen to it yourself and make sure you like it.


Best of luck in your quest!


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Huck
 

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Steve,


Huckster is right. Formulas may give you an indication of where you'll end up, but then again empirical results like these reported on here says a lot about reality. Not very encouraging:


quote: SGHT and HOME THEATER magazines recently reviewed the three projectors mentioned [below] and here is basically what they had to say:


o YAMAHA DPX-1: 10 foot lamberts light output on 8-foot wide Stewart greyhawk screen, peak white level of 16 foot lamberts on a 7-foot wide 1.3 gain screen.

o SELECO HT 200M: peak white level of 7 foot lamberts on a 7-foot wide 1.3 gain screen.

0 SELECO HT 250: light output of 8 foot lamberts on 8-foot wide Stewart Greyhawk screen.


quote: Did the Seleco 200DM really measure out at 7 ft-Lamberts on a 7' wide screen? Jeez, that means that the rough lumens output it around 300 lumens, a far cry from their 800 lumen spec. Of course Seleco is not the only offender in that category.


quote: As for those measurements on the HT 200 DM, they did include the Grayhawk, and I can guarantee you that Seleco didn't pre-load their published specs by taking them on a screen with a gain less than 1!! I tried to show with numbers what I was actually reviewing...not competing with the numbers game.


None of the published figures from any manufacturer have any relation to what we will actually experience with proper contrast and brightness adjustments in our homes...and none of the figures from different manufacturers can be compared with one another. Unless the measurement equipment is identical, and traceable to a standard (like NIST), and calibrated often; and the measurement procedure identical and followed to a 't' by everyone, and the screens identical and matched...we are in outer space when it comes to comparing manufacturers published specs. Sorry if I burst anyone's fantasies here.

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I don't mind bursting you bursting my fantasies but I would like some advice on how to make a decision. I would like to put up a 100" screen.


Based on the posts here, I am looking at the Sharp (or the new Seleco)..In other words, one of the new 16:9 DLP with the 6 color wheel. I was hoping to use a GreyHawk...


(FYI, my overall system design was published here: http://www.avsforum.com/ubb/Forum10/HTML/009519.html

asking for feedback (which I got almost none)...


I don't know how to know what the system would look like with a GreyHawk without seeing it. The only way I know how to do that is to go to a local store (in my case LaserLand in Cupertino) -- However, since I am unwilling to pay their marked up fee, I don't feel that I should be doing this. Knowing people in the business, I don't think it is cool to demo things at one place and then order somewhere else...


I would like to order from AVS or through a buddy who is an installer. If it was a few hundred dollars, it would be one thing but we are talking $3k+....


So, who do I trust about whether or not it is bright enough sight unseen? Ah, the dilema...


-Jon
 

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Discussion Starter · #13 ·
Augie,

Thanks for the spreadsheet, it will come in handy.


I guess I'll have to try out a projector and see how it handles the ambient.


--sdc
 
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